James “Whitey” Bulger is charged with a litany of crimes – including participating in 19 murders – during what prosecutors describe as a decadeslong reign of “murder and mayhem.”

But Bulger’s lawyers have spent much of their energy defending their client against something he doesn’t face criminal charges for: being a longtime FBI informant.

The defense has vehemently denied the prosecution’s claim that Bulger was an informant, going so far as to say his Irish heritage would prohibit such a thing.

They’ve also spent hours trying to discredit a 700-page FBI file that prosecutors say shows Bulger ratted on everyone from mobsters in the Italian Mafia to members of his own gang.

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The defense strategy may be coming from Bulger himself.

In “Whitey Bulger: America’s Most Wanted Gangster and the Manhunt That Brought him to Justice,” a book published this year, Boston Globe reporters Shelley Murphy and Kevin Cullen include excerpts from letters Bulger wrote to a friend from jail saying he wants to show the world that he did not kill women and he was not a rat.

“I never put one person in prison in my life,” he wrote in one of his letters.

During his opening statement to the jury, Bulger’s lead attorney, J.W. Carney Jr., referring to Bulger’s Irish descent, said becoming an informant was “the worst thing an Irish person could consider doing” because of the history of The Troubles, a violent 30-year conflict in Ireland between Catholics and Protestants that left more than 3,600 people dead.

“James Bulger never ever – the evidence will show – was an informant,” Carney said. Instead, he said, Bulger paid FBI agents to protect him from being prosecuted.

Another Bulger attorney, Hank Brennan, has focused on Bulger’s informant file, suggesting it was fabricated by former FBI Agent John Connolly, who was convicted of racketeering and second-degree murder for leaking information to Bulger and his partner, Stephen “The Rifleman” Flemmi.

Brennan spent hours last week questioning former FBI Agent John Morris about reports from other FBI informants that appeared strikingly similar to reports on information Connolly attributed to Bulger. He suggested that Morris and Connolly said Bulger gave them the information to advance their own careers at a time when cultivating informants who could help bring down the Mafia was a national priority for the FBI.

But investigators who spent years trying to build a case against Bulger say there is overwhelming evidence that Bulger spent 15 years as an informant – 1975 to 1990 – providing the FBI with information on local Mafia leaders, drug dealers and even criminals in his own South Boston neighborhood.

“I think clearly his attorney has marching orders from the defendant. He’s more obsessed with not being seen as an informant than as a mass killer, which is an absurdity,” said Thomas Duffy, a retired state police major who investigated Bulger.

“It doesn’t make any sense other than in his own mind. It just has to do with how he wants to be remembered.”

Michael Kendall, a former federal prosecutor who investigated several of Bulger’s associates, said it’s not surprising that Bulger’s trial strategy would focus on trying to deny his status as an informant.

“He’s had a good run for 83 years, and he realizes he’s not going to get out of prison. He’s not going to win the trial in terms of a guilty or innocent verdict. So he’ll try to win the trial in terms of settling scores or criticizing people he doesn’t like or making semantic distinctions on his ratting out his friends to the government,” Kendall said.

But others say the defense effort could be part of a larger strategy to highlight the unethical behavior of the FBI in the hope that the jury will be disgusted with the government and less likely to believe its case against Bulger.

“What they are doing is they are attacking the credibility of every single witness to show the breadth and the depth of the corruption within the FBI at the time he allegedly had a relationship with them,” said Suffolk University Law School professor Christopher Dearborn.

Before the trial, Carney was asked if he believes his client can receive a fair trial in Boston, where Bulger’s name and notorious reputation are so well known.

“Mr. Bulger believes that he will have a fair trial if he is able to present the whole truth concerning his relationship with the Department of Justice and FBI,” Carney responded in an email to The Associated Press, “including that he was never an informant.”